The present invention relates to a device for compensating length variations in tensioned cables, with substantially constant traction.
The conductors of overhead power lines for distributing and carrying power or for railroad traction are installed with a very specific tension value which takes into account the ultimate tensile strength of the cables, the strength of the supporting structures and the requirements in terms of maximum sag at the center of the span.
The configuration assumed by the cable after tensioning is not constant and invariable over time but is closely dependent on the ambient temperature, since the material of which the cable is made, which can be copper or alloys thereof, aluminum or alloys thereof, steel or aluminum with a steel core, expands or contracts as the temperature increases or decreases.
This fact causes the sag in the middle of the span to increase or decrease. This variation in the sag of the span, traced by the cable owing to its own weight, cannot be tolerated in some applications owing to electrical and/or mechanical reasons.
The electrical reasons consist of the fact that as the sag increases, the insulation distances towards the surrounding parts, such as for example the ground, decrease. Moreover, the decrease in tensioning force caused by an elongation of the cable makes said cable more subject to oscillations and swaying caused by the wind, which can make the cable move dangerously close to other cables of different phases or to grounded parts.
The mechanical reasons can be seen typically in the catenary curve of electric contact lines of railroads. Conventionally these lines are constituted by a supporting cable from which the contact wire is suspended; the pantograph of locomotives makes sliding contact with the underside of said wire to tap the current. The contact wire is supported by the supporting cable by means of so-called "droppers" placed at short preset distances, so that while the profile of the supporting cable has the typical sag, the profile of the contact wire remains significantly straight, so as to be practically horizontal, in order to allow correct sliding of the pantograph. A contraction of the supporting cable and/or of the contact wire, caused by thermal variations, can cause unacceptable stresses on said cables or on the supports and can cause an abnormal lifting of the contact wire with respect to the rail level; such stresses and lifting might not be compatible with correct collection of the current by the pantograph, in addition to causing abnormal wear of the contact wire. Likewise, an elongation of the supporting cable and/or of the contact wire, caused by thermal variations, can cause abnormal lowerings of the contact wire, equally causing problems in the correct collection of the current.
Different systems are used to compensate variations in the length of the cables as a consequence of temperature variations.
A first system is constituted by counterweighting and consists substantially in connecting one end of the cable, by means of pulleys, to a weight which corresponds to the tensioning force of the cable or is reduced as a function of the ratio of the pulley system. In this way, the tension of the cable remains unchanged as the length of the cable varies, since the installed weight is invariant. This system, which is still the most widely used system today, has the drawback that it requires a large space occupation, which is not always compatible with the installation requirements, especially in the railroad field, such as for example in tunnels, where very often it is physically impossible to provide recesses into which the counterweights might slide, or in which it might be possible to provide recesses but at excessively high costs. Another drawback of this system is the high installation cost, caused mainly by the need to resort to auxiliary structures for supporting, guiding and protecting the counterweights.
Another system is based on the use of a gas-filled compensator. In this system, one end of the cable is connected to a device which comprises a spring/damper unit containing compressed gas, usually nitrogen; said unit provides a significantly constant load. This system has the advantage, with respect to counterweighting, of requiring a modest space occupation for its installation, but it is not free from drawbacks, since problems are observed a few years after the first installations, mainly in relation to the loss of gas from the unit, requiring continuous monitoring and maintenance. The cost of this device is also high and restricts its use to particular cases.
Compensators are also known which are actuated by an electric motor and are substantially constituted by a device having a sensor for detecting variations in the length of the cable caused by thermal variations, which drives an electric motor which in turn actuates a rod connected to the cable to be compensated. This system, too, is not free from drawbacks, since it has a high cost, also caused by the need to install an electrical power supply and control panel and by the need to have a low-voltage electric power source, which is not always available or at least not available cheaply. Such a technical solution is disclosed in FR-A-2,456,635.
Another compensation system is constituted by the simple spring compensator. This system substantially uses a helical spring which "damps" the mechanical effects caused by the expansion or contraction of the cable but is unable to keep the tension of the cable constant due to the characteristic of the spring of varying the load in proportion to the stroke.